Please check which activity type you are using.
If the activity requires a task to be created, this could be the cause of the problem. If a task isn't created for the activity, the status will change to Evidence Submitted once a document is uploaded, but will revert back to Not Started when the evidence is assessed.
Please ensure the task is created(if required) and marked as complete in order for the activity status to update correctly.
Our guide to Learning Plan Activities can be found here: A How-To-Guide to Learning Plan activities.